As described in "Adsorption Processes for Water Treatment" by Faust and Aly, 1987, p. 370, "Groundwater represents greater than 95% of available fresh water in the United States, and this includes the Great Lakes. About 80% of all public water supplies rely on groundwater for portable water sources, and approximately 96% of all water employed for rural domestic purposes is obtained from groundwater. Usage of groundwater by public water utilities has almost tripled since 1950 from 3.5 bgd to 11 bgd in 1975. Concurrently, more and more contamination of these groundwaters by organic compounds has been reported (Table 8-18)."
The above problem is made more difficult because most current treatment methods involve a mass transfer of the organic contaminants from the water to be treated into an air stream. "Air Stripping" is the least expensive and most widely used of these processes. The contaminated air must then be released into the atmosphere or treated separately. As air pollution levels rise, Federal, State and Local air quality regulations have become more stringent and "air stripping" without air treatment is less common.
The limits of groundwater and atmospheric tolerance for pollution requires an improved process that can more economically and effectively treat contaminated water before it becomes part of the groundwater resource and to improve the treatment of contaminated water, regardless of source, before industrial or domestic use.
Many of the common organic pollutants found in contaminated water situations are subject to biological degradation into compounds that are harmless. Many organic compounds can be adsorbed on activated carbon and thus removed from water. A great number of these organic compounds are both biodegradable and adsorbable. When feasible, the biological process provides a natural method, breaking down many compounds to carbon dioxide, water, inorganic salts, and biomass. Use of carbon and activated carbon to remove odors, unwanted colors, and pollutants from water goes back about 4,000 years. There are several activated carbon adsorption processes but all suffer from the high cost of replacing or thermally regenerating the carbon at regular intervals.
Research on the growth of bacteria indicates that in low concentrations of the organic substrate, bacterial growth is favored by attachment to solid surfaces. In fact, attached bacteria will grow at organic concentrations lower than those that will support growth in the free or unattached state. This is extremely important in a biodegradation situation because the goal is to get the organic concentration in the final effluent as low as possible--preferably by biological means.
In situations where high concentrations of an organic contaminant, such as phenol, are toxic to bacteria, activated carbon can adsorb the phenol and lower the concentration to a level the bacteria can assimilate. By adding nutrients and oxygen over a period of time, the phenol can be gradually released or desorbed from the carbon and biodegraded by the bacteria attached to the carbon particles. This phenomena has been described as "Substrate-Inhibited Microbiological Regeneration of Granular Activated Carbon". Sigurdson and Robinson, The Canadian Journal of Chemical Engineering, 56, Jun. 1978, pp. 330-339. When first starting up, the carbon particles can often adsorb the organic contaminants and provide treatment while the bacterial population adapts and builds up to the maximum biodegradation contribution.